When you know two languages well, they form a continuum in your brain. You don’t need to translate between them. You don’t get their grammars confused. You ‘lean’ this way or that.
At this point I can read Dutch pretty well, and even write at the level of a child. That, despite never having studied the language, ever.
Oh, except I did attend first year elementary school, sorry. I do remember being introduced to ‘ij’, ‘ui’ and ‘eu’.
Other than that though, nothing.
Knowing two languages makes you magic. It gives you the certainty that learning another language is possible.